I. Field of the Invention
This invention relates generally to cardiac rhythm management apparatus incorporating impedance sensing capability, and more particularly to a multi-site impedance sensing system having one or more leads arranged to localize sensing of impedances in the left ventricular chamber.
II. Discussion of the Prior Art
The first use of electrical impedance to attempt to measure the chamber volume of the heart reported by Geddes, et al. in a paper entitled "Continuous Measurement of Ventricular Stroke Volume by Electrical Impedance", Cardiovascular Research Center Bulletin, Vol. 4, pp. 118-131, .COPYRGT.1966, involved the use of a pair of epicardial electrodes which were sewn to the surface of the left ventricle spanning its volume. A constant AC current was injected between these electrodes and the potential difference generated was used as a measure of the left ventricular volume. Converting the potential difference into electrical resistance, the group obtained calibration constants for this technique of 4.2-10.0 ml./ohm and found the calibration to be highly dependent on the individual. Because the electrodes were sutured into position, the separation was highly stable and there was very little signal artifact due to motion of the electrodes.
In 1970, in an effort to reduce the invasiveness of the technique, the same research group made similar measurements using a single multi-electrode catheter positioned in the left ventricle. They obtained similar results to the initial studies but the technique was only useful acutely due to the possibility of thrombus formation in the left heart and its attendant dangers. Subsequent research concentrated on acute catheter-based measurements in the left heart primarily because knowledge of the function of the left ventricle was considered to be more critical than the right ventricle for most patient conditions.
The first application of impedance plethysmography to an implantable device is described in Salo, et al. U.S. Pat. No. 5,190,035 which uses electrical impedance from a single multi-electrode lead positioned in the right ventricle to measure stroke volume. In its simplest implementation, this approach required a three-electrode lead. The current source was connected between the distal tip electrode and the pacemaker case and the measurement of impedance was made between the two remaining ring electrodes, which were positioned within the right ventricle. This arrangement was satisfactory for the measurement of stroke volume, since right and left ventricular stroke volumes must remain, on the average, equal. The system was relatively unpopular due to its requirement for a non-standard tripolar lead and was also unable to measure left ventricular parameters in situations (such as left bundle branch block, etc.) where the left ventricular performance can diverge from that of the right ventricle.
The requirement for a three electrode lead was addressed by the Hauck, et al. U.S. Pat. No. 5,036,894 which teaches the utilization of the pacemaker metal can and an additional electrode disposed on the device's insulating lead connector block as the two proximal electrodes. Thus, it was possible to make tetra polar impedance measurements using a standard bipolar pacing lead in the right ventricle, obviously, this configuration did not address the measurement of left ventricular parameters and was also sensitive to global impedance changes within the torso (such as respiration) due to the wide separation of sensing electrodes.
A further area of concern is the sensitivity of the lead system to motion. The heart is constantly in violent motion. During each cardiac cycle, there is inward and outward movement of septal and lateral walls, upward motion of the apex and spiral movement of the entire chamber wringing the blood out of the ventricles. In addition, the whole heart swings from the great vessels. These cardiac motions promote lead motions which result in impedance changes which are misinterpreted as changes in volume. Separating current sources and measuring electrodes can diminish the impact of this motion, by placing the measuring electrodes in regions of more uniform current density. The situation can also be improved by fixing the electrodes in position so that they cannot move relative to the heart. Pacing leads have been found to become fixed in position by fibrotic encapsulation in the first two weeks and measurements made before this process is completed are suspect. In fact, we have noted that about half the amplitude of impedance signals acquired immediately after implant is a result of lead motion and unrelated to ventricular volume.
The application of impedance plethysmography is additionally difficult when the goal is to make measurements in dilated hearts. Because of the large volume of these hearts (up to five or even ten times that of a normal heart) the average measured impedance is very low. In addition, the ejection fraction of these hearts may be as low as five percent resulting in a very small absolute change in impedance during the cardiac cycle. Thus, the signal/noise ratio in these patients is much worse than for patients with normal hearts. The baseline impedance, which follows the equation: Z=.rho..multidot.L.sup.2 /V (where .rho. is the resistivity of the blood, L is the distance between sensing electrodes and V is the chamber volume) can be increased by increasing the inter-electrode distance, L with significant improvement in signal/noise ratio.
The subject of this application is an impedance plethysmography system to meet the multiple requirements of an implantable system. These requirements are that it 1) utilize simple and, if possible, standard leads and not require leads in addition to those necessary for therapy application, 2) measure left ventricular cardiac function, 3) be relatively immune to motion of the heart permitting accurate measurement from the time of system implant and be usable in dilated chambers, such as those in patients with moderate to severe heart failure, with inherently poor signal/noise ratio.